This disclosure relates generally to hydrocarbon-producing wells, and more particularly to apparatus and methods for inspecting integrity of a multi-barrier well bore with respect to formation of defects in different material layers around the wellbore.
In hydrocarbon-producing wells such as oil and gas wells (including pipelines), it is important to ensure that there is no oil or gas leakage from the well into the surrounding rock formation. This is a critical safety requirement for the hydrocarbon-producing wells. The obvious consequences of blowouts or leaks include material damage, personnel injuries, loss of production and environmental damages, resulting in costly and risky repairs.
To ensure well safety, the wellbore is typically constructed with multiple layers, each layer includes a well casing made of metal, usually steel, surrounded by a cement wall, also referred herein as cement annuli. At shallow surface depths the number of layers are more. Some defects can occur during completion of a well construction, for example fluid channel defects are formed sometimes when cement is not replacing all mud/oil that is present in the well during well completion operation. Once the hydrocarbon-producing well has been commissioned for operation, the well integrity requires to be monitored, as defects like cracks, cement de-bonding etc. start occurring during an operational life of the hydrocarbon-producing well.
Some of the reasons for formation of these defects include, for example, a thermal expansion of cement during the initial process of formation of the cement wall, that expands the well casing, and subsequent cooling and contraction of cement during the setting process which leaves an annulus or micro-annulus i.e. a small air gap, at the steel-cement interface. Alternately, sometimes, during drilling, the hydrostatic pressure in the well casing reduces, leading to contraction of the well casing. This can result in creation of a defect in the casing cement interface. Further, an outer surface of the well casing may be covered by an oil film or a corrosion inhibitor which may not allow proper bonding with cement and a defect may be formed due to improper bonding. Moreover, cyclic pressure and temperature variations during hydrocarbon production also lead to the de-bonding of cement from the well casing. Other wellbore defects may include pitting/material loss of metal casings, fluid channel defects in the cement annuli (e.g. oil based mud defects), eccentricity of casings resulting in uneven cement distribution, etc.
It is important to monitor and detect these defects impacting the integrity of the wellbore, in order to ensure a safe operation of the well. Some of the techniques to detect these defects include use of acoustic waves in the ultrasound region that travel through the wellbore and casing-cement interfaces and are reflected back. The reflected waves are studied for their attenuation, amplitude, impedance, time of flight to determine the presence of annulus and the extent of penetration of the annulus into the cement wall.
The limitation of the ultrasound based technique being used is that, the oil and other well fluids, and cement signatures for attenuation are similar, and therefore it is difficult to accurately determine the presence of defects, or the extent of damage. Also, ultrasound wave of a particular frequency may not be able to pass through all the layers, thereby limiting its use when multiple layers are present between the transmitter and receiver. Further some of these techniques employ complex statistical signal processing, making it a complex procedure.
There are some other techniques that use X-rays and Gamma rays based reflections to detect the defects, but they face limitations related to space constraints of the wellbore, and it is a challenge to have the source and detector configurations that fit in an inspection tool that can traverse the wellbore in a high pressure and a high temperature environment and give reliable and discernable images from which defects can be detected.